1. Field of the Invention
This invention relates to a catalyst for purifying the exhaust gas of an automobile. More particularly, it is a catalyst which can remove NO.sub.x very effectively from even the combustion product of a lean fuel mixture containing an excess of oxygen. It also relates to a method of producing such a catalyst.
2. Description of the Prior Art
The catalysts which can oxidize carbon monoxide (CO) and hydrocarbons (HC) and reduce nitrogen oxides (NO.sub.x) simultaneously are widely used for purifying the exhaust gases of automobiles. These catalysts basically comprise a support formed from a refractory material, such as cordierite, and carrying a coated and baked layer of a slurry of .gamma.-alumina, and a metal such as Pd, Pt or Rh, or a mixture of such metals carried thereon. A great many proposals have hitherto been made to obtain catalysts of high activity. For example, Japanese Patent Application laid open under No. 11147/1986 discloses an improvement in the catalyst of the type which comprises a noble metal distributed on particles of .gamma.-alumina stabilized with an oxide of a rare earth element. The improved catalyst comprises rhodium distributed on particles which are substantially free from any oxide of a rare earth element.
The purifying characteristics of the catalysts which have hitherto been used or proposed, however, depend largely on the air-fuel ratio of the fuel mixture which is employed in an engine. If it is a lean mixture, the catalyst exhibits a high activity of oxidation but a low activity of reduction, as the combustion product contains a large amount of oxygen (O.sub.2) On the other hand, the catalyst exhibits a high activity of reduction but a low activity of oxidation, if a rich fuel mixture is employed. The catalyst works most effectively when the fuel mixture has a theoretical air-fuel ratio (A/F=14.6) at which its activities of oxidation and reduction are balanced. In an automobile equipped with an exhaust gas purifier employing a catalyst, therefore, it is usual to detect the amount of oxygen in the exhaust gas and feed back the results to control the air-fuel ratio of the fuel mixture to maintain it at or near the theoretical level which has hereinabove been stated.
The users of automobiles, however, also desire to reduce the consumption of fuel. It is known that their desire can be met if they employ a fuel mixture containing an excess of oxygen during their ordinary travel. If such a lean fuel mixture is employed, however, O.sub.2 adsorbed by the catalyst prevents NO.sub.x in the exhaust gas from contacting the active metal on the catalyst and make it difficult to reduce NO.sub.x, though HC and CO can be removed by oxidation. Therefore, a catalyst which comprises a transition metal, such as Cu, carried on zeolite by ion exchange has been proposed for use in the purification of exhaust gas from an engine in which a lean fuel mixture is employed.
Zeolites are crystalline aluminosilicates which are represented by the general formula: EQU xM.sub.2 /n.Al.sub.2 O.sub.3.ySiO.sub.2
as is well known in the art. There are commercially available many varieties of zeolites characterized by crystal structures having different channels or pore diameters which depend on M (a metal having a valency of n), x and y. They have a high cation exchange capacity, since they have the tendency to retain in the crystals cations, such as Na.sup.+ and K.sup.+, which make up for the insufficiency of a positive charge which is due to the substitution of Al.sup.3+ for a part of Si.sup.4+.
Japanese Patent Application laid open under No. 25250/1985 discloses a catalyst for the catalytic decomposition of nitrogen oxides which contains copper ions in a crystalline aluminosilicate having an appropriate spacing of lattice planes (d value) as determined by X-ray diffraction and having a molar ratio of SiO.sub.2 :Al.sub.2 O.sub.3 of 20 to 100:1, and a method of using it.
Japanese Patent Application No. 291258/1987, which was filed by the assignee of the inventors of this invention, proposes a catalyst for the purification of exhaust gas which comprises zeolite ionically exchanged with a transition metal and carried on a refractory support. The preferred transition metals are Cu, Co, Cr, Ni, Fe and Mn. Copper is the most preferable of all.
Zeolites have fine pores with a size of several .ANG.ngstrom units which is comparable with the size of a molecule, as they are called molecular sieves. Hydrocarbons are selectively caught in those pores. As the transition metal which has been introduced by ion exchange forms active sites in the pores, the hydrocarbons are adsorbed therein and react with nitrogen oxides. Therefore, it is possible to remove NO.sub.x effectively from even the combustion product of a lean fuel mixture.
However, zeolites are available in a wide variety of types differing in structure from one another and each type of zeolite has a variety of coordination arrangements. Therefore, the performance of a catalyst for purifying exhaust gas depends on the type of zeolite which is employed and its coordination arrangement, even if copper may be employed as the transition metal. No due consideration has hitherto been given to the nature of the active sites in a catalyst for purifying exhaust gas, but all of the known catalysts comprising zeolite and a transition metal have been unsatisfactory in performance.